docs: def add_authorized_key(public_key, application_name, user=None):
      """Add given key to the authorized_key file.
  
      :param public_key: Public key.
      :type public_key: str
      :param application_name: Name of application eg nova-compute-something
      :type application_name: str
      :param user: The user that the ssh asserts are for.
      :type user: str
      """
      with open(authorized_keys(application_name, user), 'a') as keys:
          keys.write("{}\n".format(public_key))